Incorporate aah changes to AudioBufferProvider into VideoEditor

This is a squashed merge from master-tungsten of the following change:

commit e1bec6e10c8d7462a0970568153a2d9db1463e6e
Author: Jason Simmons <jsimmons@google.com>
Date:   Thu Jun 16 14:22:57 2011 -0700

    Change the interface of the AudioMixer and AudioBufferProvider to accept a presentation timestamp

    Change-Id: I565b48450f98089767bf92b435826c9f4baeb01c

Change-Id: I76021f5de9aad92381678e299c8e8b5a77d577bf
Signed-off-by: Mike J. Chen <mjchen@google.com>
Signed-off-by: John Grossman <johngro@google.com>
Signed-off-by: Jason Simmons <jsimmons@google.com>